Heavy olefin production process

ABSTRACT

A process is presented for the selective separation and recovery of large normal paraffins from a heavy kerosene boiling point fraction. The process includes passing the heavy kerosene fraction through an adsorption separation system for separating the normal paraffins from the paraffin mixture. The recovered extract and raffinate streams are mixed with a diluent made up of a lighter hydrocarbon. The subsequent diluted extract and raffinate streams are passed through first fractionation columns to separate the desorbent from the diluent and the heavier paraffins. The mixture of the diluent and heavier paraffins is passed through a second set of fractionation columns to separate the diluent and the heavier paraffins.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.12/272,151 filed Nov. 17, 2008, the entire contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates to the separation of lightly branched and normalparaffins from a hydrocarbon mixture and the formation of largerolefins. Specifically, the separation is of larger lightly branched andnormal paraffins from a heavy hydrocarbon mixture to form olefins, whichrequires a combination of adsorption separation and distillation.

BACKGROUND OF THE INVENTION

The production of normal paraffins provides the ability of upgradingproducts from straight runs of hydrocarbon streams derived from crudeoil fractionation. In particular, straight run kerosene is furtherprocessed to separate out normal paraffins for higher valued products,such as used in the production of linear alkyl benzenes (LAB). Normalparaffins in the range of C10 to C13 are important precursors to LABproduction, which is in turn used to produce linear alkyl benzenesulfonate (LAS). LAS is the predominant surfactant used in theproduction of detergents.

The large utility of detergents and other cleaners has led to extensivedevelopment in the areas of detergent production and formulation. Whiledetergents can be formulated from a wide variety of different compoundsmuch of the world's supply is formulated from chemicals derived fromalkylbenzenes. The compounds are produced in petrochemical complexes inwhich an aromatic hydrocarbon, typically benzene, is alkylated with anolefin of the desired structure and carbon number for the side chain.Typically the olefin is actually a mixture of different olefins forminga homologous series having a range of three to five carbon numbers. Theolefin(s) can be derived from several alternative sources. For instance,they can be derived from the oligomerization of propylene or butenes orfrom the polymerization of ethylene. Economics has led to the productionof olefins by the dehydrogenation of the corresponding paraffin beingthe preferred route to produce the olefin.

The choice of carbon numbers is set by the boiling point range ofstraight run cuts from crude distillation. Kerosene boiling rangefractions from crude oil provide heavier paraffins. Paraffins having 8to 15 carbons are present in significant concentrations in relativelylow cost kerosene. These paraffins have been a predominant source forlinear alkanes and the leading source of olefin precursors for use inmaking LABs. Recovery of the desired normal paraffins from kerosene isperformed by adsorption separation, which is one process in overallproduction of LABs. The paraffins are then passed through a catalyticdehydrogenation zone wherein some of the paraffins are converted toolefins. The resultant mixture of paraffins and olefins is then passedinto an alkylation zone in which the olefins are reacted with thearomatic substrate. This overall flow is shown in U.S. Pat. No.5,276,231, which is incorporated by reference in its entirety, directedto an improvement related to the adsorptive separation of byproductaromatic hydrocarbons from the dehydrogenation zone effluent. PCTInternational Publication WO 99/07656 indicates that paraffins used inthis overall process may be recovered through the use of two adsorptiveseparation zones in series, with one zone producing normal paraffins andanother producing mono-methyl paraffins.

Adsorptive separation on a large scale does not allow for the moving ofthe adsorption bed, therefore the technology uses simulated moving bedtechnology. The simulation of a moving adsorbent bed is described inU.S. Pat. No. 2,985,589 (Broughton et al.), which is incorporated byreference in its entirety. The success of a particular adsorptiveseparation is determined by many factors. Predominant among these arethe composition of the adsorbent (stationary phase) and desorbent(mobile phase) employed in the process. The remaining factors arebasically related to process conditions, which are very important tosuccessful commercial operation.

While adsorption separation technology allows for the separation ofnormal paraffins from a hydrocarbon mixture, there are problems inrecovering higher molecular weight paraffins after the separation thatcurrently limit the ability to recover higher molecular weight normalparaffins.

SUMMARY OF THE INVENTION

The present invention provides a process to recover normal and lightlybranched olefins from a heavy cut of the kerosene or diesel boilingfraction that was generated in the course of dehydrogenation of afeedstock consisting primarily of normal and lightly branched paraffins.The process comprises using an adsorption separation unit to separatenormal and lightly branched olefins in the C15 to C28 range from theremainder of the heavy kerosene or diesel. The normal and lightlybranched olefins are passed out in an extract stream which includes thedesorbent. The remainder is passed out in a raffinate stream, which alsoincludes the desorbent. The extract stream is mixed with a diluent,thereby creating an intermediate extract stream. The intermediateextract stream is passed to a first fractionation column. The diluentprovides for an intermediate boiling point liquid to allow the operationof the first fractionation column at a temperature low enough to preventcracking of the normal and lightly branched olefins. The firstfractionation column creates an overhead stream comprising thedesorbent, and an intermediate extract bottoms stream comprising thenormal and lightly branched olefins and the diluent. The raffinatestream is mixed with the diluent, creating an intermediate raffinatestream, and passed to a second fractionation column, that is operated ata temperature low enough to prevent cracking of the larger hydrocarbonsin the intermediate raffinate stream. The second fractionation columnhas an overhead stream comprising the desorbent and an intermediateraffinate bottoms stream comprising the heavy hydrocarbons and thediluent.

The intermediate extract bottoms stream is passed to a thirdfractionation column, which is operated at below atmospheric pressure toallow boiling of the normal and lightly branched olefins at atemperature below cracking temperatures. The third fractionation columnhas an overhead comprising the diluent, and a bottoms stream comprisingthe normal and lightly branched olefins in the C15 to C28 range.

The intermediate raffinate bottoms stream is passed to a fourthfractionation column, which is operated below atmospheric pressure toallow boiling of the heavy hydrocarbons at a temperature below crackingtemperatures. The fourth fractionation column has an overhead comprisingthe diluent, and a bottoms stream comprising the heavy hydrocarbonsseparated from the normal paraffins.

The process provides for the recovery of normal and lightly branchedolefins from a heavy hydrocarbon stream, while allowing separation andrecovery of the desorbent at temperatures sufficiently low to preventthe destruction through cracking of the desired product stream of normaland lightly branched olefins. The desorbent and diluent are recoveredand recycled in this process.

Additional objects, embodiments and details of this invention can beobtained from the following drawing and detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIGURE is a diagram of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The recovery of higher molecular weight paraffins from the kerosenefraction presents many problems. In particular, the recovery of normalparaffins in the C15 to C19 range presents several conflicting problems.The feedstock of choice is a heavy cut from the kerosene boilingfraction. The extract stream from an adsorption separation unit willcontain the normal paraffins, and the desorbent from the process. Thecurrent recovery of the normal paraffins in the C8 to C14 range from theextract mixture containing the desorbent and the normal paraffins is tofractionate the mixture. The desorbent is recovered in the fractionationcolumn overhead, while the normal paraffins are recovered from columnbottoms. With higher molecular weight paraffins in order to separate theparaffins from the desorbent, sufficient heat must be applied to boilthe normal paraffins. However, at those temperatures the normalparaffins will crack, and will render the separation processineffective. If the fractionation is performed under a vacuum, in orderto reduce the temperature of operation, then the desorbent can not becondensed in a manner that does not require expensive technology, suchas a significant refrigeration system.

In addition, large olefins, i.e. olefins having 10 or more carbons, wereused for a variety of purposes. One important purpose was thedevelopment of detergents and surfactants. However, it was found thatolefins that were highly branched presented problems. Highly branchedolefins were typically highly branched, and were characterized by alarge portion of the aliphatic carbon chain is in at least one branchand more commonly in three or more alkyl group branches. Thus, thebranched olefins have a relatively large number of primary carbon atomsper aliphatic alkyl group. Another characteristic of highly branchedolefins is that the double bond is not on the second carbon atom of thelongest chain, but is distributed among the branched olefins atdifferent positions. When alkylating highly branched olefins withbenzene, a low selectivity to 2-phenyl alkanes is obtained with aselectivity of 2-phenyl alkanes typically between 10 and 40. Anothercharacteristic of highly branched olefins is the presence of internalquaternary carbon atoms, with the proportion of internal quaternarycarbon atoms near 10 mol % of the carbons.

It has been found that, while linear alkylbenzenes are best for avariety of applications, modified alkylbenzenes can in many instances bequite effective. Modified alkylbenzenes are formed from benzenealkylated with lightly branched olefins. Therefore, it has been foundthat separating out lightly branched olefins from highly branchedolefins is an important aspect for increasing the available stock ofuseful olefins. The characteristics of lightly branched olefins includethe number of primary carbon atoms as between that of highly branchedolefins and linear olefins. Another characteristic of lightly branchedolefins is that a high proportion of the olefins have the double bond onthe second carbon atom and that the proportion will be between 40 and100 percent. A third characteristic of lightly branched olefins is thelow proportion of internal quaternary carbon atoms.

The lightly branched monoolefin may be an alpha monoolefin or avinylidene monoolefin, but is normally an internal monoolefin. As usedherein, the term “alpha olefins” refers to olefins having the chemicalformula, R—CH═CH2. The term “internal olefins,” as used herein, includesdi-substituted internal olefins having the chemical formula R—CH═CH—R;tri-substituted internal olefins having the chemical formulaR—C(R)═CH—R; and tetra-substituted olefins having the chemical formulaR—C(R)═C(R)—R. The di-substituted internal olefins include beta internalolefins having the chemical formula R—CH═CH—CH3. As used herein, theterm “vinylidene olefins” refers to olefins having the chemical formulaR C(R)═CH2. In each of the preceding chemical formulas in thisparagraph, R is an alkyl group that may be identical to or differentfrom other alkyl group(s), if any, in each formula. Insofar as permittedby the definition of the term “internal olefin”, when the lightlybranched monoolefin is an internal monoolefin, any two carbon atoms ofthe aliphatic alkenyl chain may bear the double bond.

The lightly branched olefins can include having two or more branches,but preferably, the lightly branched olefins comprises a mixture thatincludes monomethyl, dimethyl, monoethyl, and monopropyl olefins with asmall amount of olefins having larger branches or more branches.Generally, it is preferred that the lightly branched olefins comprise atleast 70 mol % of the olefins in the mixture with a more preferredcomposition comprising at least 85 mol % of the olefins in the mixture.Monoolefins having either two alkyl group branches or four primarycarbon atoms and a quaternary carbon atom comprise generally less than10 mol-%, and preferably less than about 1 mol-%, of the total lightlybranched monoolefins in the mixture.

The present invention provides for overcoming these two competingdrawbacks. The process comprises adding a diluent to the extract andraffinate streams leaving the adsorption separation unit. The extractand raffinate streams are then fractionated to separate the desorbentfrom the extract and raffinate streams, and then pass the remainingmixtures to a fractionation system operated at low pressure andtemperature to separate the diluent from the extract and raffinatestreams.

In particular, the process for producing a heavy paraffin product streamfrom a hydrocarbon feedstream is shown in the FIGURE. The processcomprises passing the hydrocarbon feedstream 12 to an adsorptionseparation unit 10. The adsorbent chosen will have pore sizes to allowfor lightly branched paraffins, as well as n-paraffins. The hydrocarbonfeedstream 12 is selected for its content of normal and lightly branchedparaffins in the C15 to C28 range. The adsorption separation unit 10creates an extract stream 14 comprising normal and lightly branchedparaffins in the C15-C28 range and a desorbent. The adsorptionseparation unit 10 also creates a raffinate stream 16 comprisingnon-normal paraffins and the desorbent. The non-normal paraffins willcomprise, generally, highly branched paraffins, and for convenience theuse of the term non-normal paraffins will refer to highly branchedparaffins. The extract stream 14 is mixed with a diluent 18, therebycreating an intermediate extract stream 22, and the raffinate stream 16is mixed with the diluent 24, thereby creating an intermediate raffinatestream 26. The adsorption separation unit 10 has as a part of the systema rotary valve system 20 which controls the positions of the inflows toand outflows from the adsorption separation unit 10.

In addition, the adsorption separation system includes a flush zone. Theflush zone is buffer zone, that moves through the system like the otherzones, and is between the adsorption zone and the desorption zone. Theflush zone clears out the material from the feed stream that has notbeen adsorbed before the desorbent zone passes. It also keeps thedesorbent from breaking through to the adsorption zone, where if thedesorbent were to break through to the adsorption zone, the processwould experience a loss of the lightly branched and normal paraffinproduct. The flush zone has a hydrocarbon that will not displace theadsorbed material while it is displacing the non-adsorbed portion of thehydrocarbon feedstream. The flush zone hydrocarbon will preferably be anintermediate non-normal hydrocarbon, such as isooctane, or an aromaticcompound in the C8 to C10 range, but can also comprise non-normalhydrocarbons outside the C8 to C10 range and can include naphthenes andaromatics. Examples of flush zone streams include isooctane and mixturesof isooctane and xylenes. One such mixture is a 70/30 mixture ofisooctane and p-xylene.

The intermediate extract stream 22 is passed to a first separationcolumn 30, which is operated at desorbent separation conditions tocreate a first column overhead stream 32 comprising the desorbent. Thefirst separation column 30 also has a first column bottoms stream 34comprising the normal and lightly branched paraffins in the C15-C28range and the diluent. The first column bottoms, or intermediate extractbottoms, stream 34 is passed to a third column 50, which is operated atdiluent separation conditions. The third column 50 has a third columnoverhead stream 52 comprising the diluent. The third column 50 alsoproduces a third column bottoms stream 54 comprising normal and lightlybranched paraffins in the C15-C28 range, which is the product stream.

The intermediate raffinate stream 26 is passed to a second separationcolumn 40, which is operated at desorbent separation conditions tocreate a second column overhead stream 42 comprising the desorbent. Thesecond separation column 40 also has a second column bottoms stream 44comprising the non-normal paraffins and the diluent. The, second columnbottoms, or intermediate raffinate bottoms, stream 44 is passed to afourth column 60 which is operated at diluent separation conditions. Thefourth column 60 has a fourth column overhead stream 62 comprising thediluent. The fourth column 60 also produces a fourth column bottomsstream 64 comprising non-normal paraffins in the C15-C28 range, andwhich is passed to other processing units in a petrochemical plant.

The columns 30, 40, 50, 60 are fractionation columns, and operatingconditions include operating at temperatures and pressures to boil, orvaporize, the liquid at the bottom of the column to create an upflowingvapor stream, and to condense the vapor at the top of the column tocreate a downflowing liquid stream.

The process further comprises passing the third column bottoms stream 54to a paraffin stream separation unit 70. The paraffin stream separationunit 70 separates the lightly branched and normal paraffins in thebottoms stream 54 into three streams: a first paraffin stream 72comprising C15 to C19 lightly branched and normal paraffins; a secondparaffin stream 74 comprising C20 to C23 lightly branched and normalparaffins; and a third paraffin stream 76 comprising C24 to C28 lightlybranched and normal paraffins. The three paraffin streams 72, 74, 76 arepassed to a dehydrogenation unit 80 a, 80 b, 80 c to convert theparaffins to olefins, generating a first olefin stream 82 comprising C15to C19 lightly branched and normal olefins, a second olefin stream 84comprising C20 to C23 lightly branched and normal olefins, and a thirdolefin stream 86 comprising C24 to C28 lightly branched and normalolefins. Each stream can be sent to a separate dehydrogenation unit 80a, 80 b, 80 c, or the process can be controlled to send each paraffinstream to a single dehydrogenation unit 80, where each stream is sentseparately, and the dehydrogenation unit 80 operating conditions areadjusted to accommodate the different feeds.

The paraffin separation unit 70 can comprise a fractionation system forgenerating the three paraffin streams, 72, 74, 76, and can include asingle divided wall column or a pair of columns. The separation of theheavy paraffins into smaller ranges of molecular weight distributionsfacilitates the dehydrogenation process. The dehydrogenation process ofparaffins operates best when the paraffins have a relatively narrowrange of molecular weights, or a more limited carbon number range. Theheavy paraffin stream is therefore divided into three streams havingnarrower ranges of carbon numbers.

The olefin streams 82, 84, 86 leaving the dehydrogenation unit 80 a, 80b, 80 c, are passed to an olefins separation unit 90 to generate anolefins stream 92 and a paraffins stream 94 comprising the unconvertedparaffins. The olefins separation unit 90 can be a single separationunit, or each olefins stream 82, 84, 86 can be passed to a separateolefins separation unit 90 a, 90 b, 90 c. The olefins separation unit 90can comprise a second adsorption separation system, wherein an extractstream 92 comprises the olefins and a raffinate stream 94 comprises theunconverted paraffins. The functions and properties of adsorbents anddesorbents in the chromatographic separation of liquid components arewell-known, and reference may be made to U.S. Pat. No. 4,642,397, whichis incorporated herein, for additional description of these adsorptionfundamentals. An adsorbent for the second adsorption separation columncomprises silicalite. Silicalite is well described in the literature. Itis disclosed and claimed in U.S. Pat. No. 4,061,724 issued to Grose etal. A more detailed description is found in the article, “Silicalite, ANew Hydrophobic Crystalline Silica Molecular Sieve,” Nature, Vol. 271,Feb. 9, 1978 which is incorporated herein by reference for itsdescription and characterization of silicalite. Silicalite is ahydrophobic crystalline silica molecular sieve having an MFI typestructure of intersecting bent-orthogonal channels formed with twocross-sectional geometries, 6 Å circular and 5.1-5.7 Å elliptical on themajor axis. This gives silicalite great selectivity as a size selectivemolecular sieve. Another adsorbent, and a preferred adsorbent, is NaXzeolite. NaX has good adsorptive properties for olefins in anolefin/paraffin mixture.

The process of olefins separation from the paraffins in the olefinsprocess streams 82, 84, 86 uses an adsorption-separation process togenerate the olefins product streams 92 a, b, c, and the raffinatestreams 94 a, b, c. The process uses a second desorbent to displace theselectively adsorbed material from the adsorbent. The desorbent isselected from a lighter hydrocarbon, preferably in the C5 to C8 range,that readily facilitates desorbing the adsorbed material. One preferreddesorbent for the second desorbent is mixture of C6 compounds, and inparticular a mixture of normal hexane and normal hexene. The extractstreams 92 a,b,c, and the raffinate streams 94 a,b,c include desorbentwith the materials being separated.

The extract streams, and the raffinate streams contain material, thedesorbent, that presents the same problem as described above regardingthe separation of lightly branched and normal paraffins from highlybranched non-normal paraffins in the paraffin feedstock. The highermolecular weight paraffins and olefins need to be separated from thedesorbent, and this process is now completed by repeating the aboveprocedure to separate the low molecular weight desorbent from the highmolecular weight olefins and paraffins. To each extract 92 a,b,c, andraffinate 94 a,b,c, stream a diluent is added after leaving theadsorption separation unit 90 a,b,c. The diluents comprises anintermediate boiling point liquid. The extract and raffinate streams arethen fractionated to separate the desorbent from the extract andraffinate streams, and then pass the remaining mixtures to afractionation system operated at low pressure and temperature toseparate the diluent from the extract and raffinate streams, thusallowing for separation at sufficiently low temperatures to preventcracking of the desired product streams. Depending on the size, andthroughput, of the hydrocarbons, the fractionation systems can beseparate, or there can be a single system with storage for processingthe different streams.

The normal operation of a fractionation column can be at atmosphericpressure or higher, but where the temperatures required to perform thefractionation are too great, the pressure can be reduced to belowatmospheric. The operation of the first and second separation columns30, 40 are operated at or above atmospheric conditions. These conditionsallow for the condensation of the desorbent during the separationprocess. The operating conditions include temperatures between 35° C.and 300° C., and pressures between 100 kPa and 500 kPa. Considerationsinclude the ability to condense the desorbent to create a reflux stream,and to boil the other components to create a vapor stream flowing upwardfrom the bottom of the columns. The temperature must be keptsufficiently low as to prevent thermal cracking of the largerhydrocarbon paraffins.

The larger hydrocarbon olefins and paraffins, in the C15-C28 rangerequire much higher temperatures to vaporize at atmospheric conditions,or above. The third and fourth separation columns 50, 60 are operated atlower pressures to allow the boiling of larger hydrocarbons at lowertemperatures. The operating conditions allow for the condensation of thediluent during the separation process. The operating conditions includetemperatures between 35° C. and 300° C., and pressures between 10 kPaand 100 kPa.

In a preferred embodiment, the invention seeks to obtain normal andlightly branched paraffins in a more narrow range. The preferred rangeis for normal and lightly branched paraffins in the C15 to C19 range,and the extract stream 14 will comprise normal paraffins in the C15 toC19 range with the desorbent. When the desorbent is separated from thenormal paraffins, the product stream will comprise a normal and lightlybranched paraffin mixture with properties listed in table 1.

TABLE 1 Properties of normal paraffin product stream sulfur content(μg/g) ≦3 bromine index (mgBr/100 g) ≦25 Saybolt color +30 aromatic massfraction (%) ≦0.4 average molecular weight 240-245 normal and lightlybranched paraffin ≧98.5 content (mass frac. %) C15 and below (mass frac.%) ≦0.5 C20 and above (mass frac. %) ≦0.5

The diluent is provided to enable separation in the first and secondfractionation columns 30, 40 at temperatures that will not thermallycrack the paraffins in the extract or the raffinate streams. A preferreddiluent comprises a hydrocarbon mixture of intermediate range paraffinshaving carbons in the C10 to C14 range. An alternate diluent cancomprise aromatic compounds, or a mixture of aromatic compounds andlighter paraffins in the C8 to C10 range, or lighter paraffins. Onechoice of diluent is an aromatic mixture comprising xylenes, and inparticular paraxylene. Another choice of diluent includes isooctane.

The desorbent for the adsorption separation process preferably comprisesparaffins in the C5 to C8 range. The smaller paraffins readily displacethe larger normal paraffins during the desorption stage of theadsorption separation process. Preferably the lighter paraffins arenormal paraffins. One preferred desorbent is n-pentane or n-hexane.Mixtures of light paraffins are also preferred, and mixtures include alight normal paraffin with isooctane. Such mixtures include a desorbentsuch as n-pentane and isooctane, or n-hexane and isooctane. Thedesorbent recovered in the first and second fractionation columns 30, 40can be recycled and passed back to the adsorption separation unit 10.

The preferred desorbent is a mixture of a light normal paraffin and anintermediate non-normal paraffin, such as n-pentane and isooctane. Thenormal paraffin is used to displace the adsorbed large normal paraffinextracted from the hydrocarbon feedstream 12 in the adsorptionseparation unit 10. A mixture is preferred utilizing a larger non-normalparaffin to minimize the chamber pressure of the adsorption separationunit 10. The mixture comprises a normal paraffin content between 1 wt %and 99 wt % with the remainder comprising a non-normal paraffin. In onepreferred mixture, the composition is a 60/40 mix of a normal paraffin,such as n-pentane or n-hexane, and a branched paraffin such asisooctane.

In addition to the continuous adsorption separation system normallycontemplated, other adsorption separation system processes are alsoapplicable for use in the present invention, and it is intended that theinvention covers other adsorption separation systems. One such systemcan be found in U.S. Pat. Nos. 3,422,005 (W. F. Avery) and 4,350,593(Andrija Fuderer) which are incorporated by reference in their entirety.This process comprises passing a hydrocarbon feedstream having normalparaffins over an adsorbent at isobarometric conditions. The normal andlightly branched paraffins are allowed to adsorb, and the adsorptionstep is followed by a purge step, typically flowing a normal paraffin,such as n-hexane, in a co-current manner to purge the adsorption chamberof the non-normal hydrocarbons. The purge step is followed by adesorption step where a desorbent is flowed counter-currently to desorbthe normal paraffins, thereby creating an extract stream comprisingnormal paraffins and the desorbent, which is then further processed asdescribed above.

In another embodiment, the invention further comprises passing therecovered diluent back into the process. The third column overheadstream 52 can be recycled and passed back to be mixed with the extractstream 14. The overhead stream 52 can be mixed with additional diluentto form the stream 18 that is mixed with the extract stream 14 to makeup for diluent lost to other process streams. The fourth column overheadstream 62 can be recycled and passed back to be mixed with the raffinatestream 16. The overhead stream 62 can be mixed with makeup diluent toform the diluent stream 24 which is mixed with the raffinate stream 16.In an alternative, the two recovered diluent streams 52, 62, can bemixed prior to mixing with the extract stream 14 or the raffinate stream16. This depends on the losses and relative amounts needed forperforming the separations.

The severity of separation of the desorbent from either or both theextract stream and the raffinate stream can require subsequentseparation. The overhead streams from the first and second columns 30,40 can comprise a mixture of the desorbent and the diluent. Thedesorbent can require further processing for obtaining a sufficientlypure desorbent for the adsorption separation unit 10. The overheadstreams 32, 42 from the first and second columns 30, 40 be collectedpassed to a fifth separation column (not shown). The fifth separationcolumn can further separate the mixture into a fifth column overheadstream comprising a light hydrocarbon, or the desorbent. The fifthseparation column can further produce a bottoms stream comprising anintermediate hydrocarbon, or the diluent.

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. A process for producing a heavy olefin product stream, comprising:passing a hydrocarbon feedstream comprising paraffins in the C15-C28range to an adsorption separation unit, thereby creating an extractstream, comprising normal and lightly branched paraffins in the C15-C28range and a first desorbent and a raffinate stream, comprising nonnormal paraffins in the C15-C28 range and a first desorbent; mixing theextract stream with a diluent, thereby creating an intermediate extractstream; mixing the raffinate stream with a diluent, thereby creating anintermediate raffinate stream; passing the intermediate extract streamto a first separation column operated at first desorbent separationconditions, thereby creating first column overhead stream comprising thefirst desorbent, and a first column bottoms stream comprising the normaland lightly branched paraffins in the C15-C28 range and the diluent;passing the intermediate raffinate stream to a second separation columnoperated at first desorbent separation conditions, thereby creating asecond column overhead stream comprising the first desorbent, and asecond column bottoms stream comprising non-normal paraffins and thediluent; passing the first column bottoms stream to a third separationcolumn operated at diluent separation conditions, thereby creating athird column overhead stream comprising the diluent, and a third columnbottoms stream comprising the normal and lightly branched paraffins inthe C15-C28 range; passing the second column bottoms stream to a fourthseparation column operated at diluent separation conditions, therebycreating a fourth column overhead stream comprising diluent, and afourth column bottoms stream comprising non-normal paraffins; passingthe third column bottoms stream to a paraffin stream separation unit tocreate a first paraffin stream comprising C15 to C19 lightly branchedparaffins, a second paraffin stream comprising C20 to C23 lightlybranched paraffins, and a third paraffin stream comprising C24 to C28branched paraffins; and passing the first paraffin stream to adehydrogenation unit to generate a first effluent stream comprising C15to C19 lightly branched olefins and unconverted paraffins; passing thesecond paraffin stream to the dehydrogenation unit to generate a secondeffluent stream comprising C20 to C23 lightly branched olefins andunconverted paraffins; and passing the third paraffin stream to thedehydrogenation unit to generate a third effluent stream comprising C24to C28 lightly branched olefins and unconverted paraffins.
 2. Theprocess of claim 1 wherein the paraffin stream separation unit comprisesa fractionation system.
 3. The process of claim 1 further comprisingpassing the first effluent stream to the second adsorption separationunit thereby creating a C15 extract stream comprising C15 to C19 olefinsand a C15 to C19 raffinate stream comprising paraffins.
 4. The processof claim 1 further comprising passing the first effluent stream to thesecond adsorption separation unit thereby creating C20 extract streamcomprising C20 to C23 olefins and a C20 to C23 raffinate streamcomprising paraffins.
 5. The process of claim 1 further comprisingpassing the first effluent stream to the second adsorption separationunit thereby creating a C24 extract stream comprising C24 to C28 olefinsand a C24 to C28 raffinate stream comprising paraffins.
 6. The processof claim 1 wherein the diluent recovered from the third separationcolumn is mixed with the extract stream.
 7. The process of claim 1wherein the diluent recovered from the fourth separation column is mixedwith the raffinate stream.
 8. The process of claim 1 wherein theparaffins in the extract stream comprise normal and lightly branchedparaffins in the C15 to C19 range.
 9. The process of claim 1 wherein thediluent is a hydrocarbon in the C10 to C14 range selected from the groupconsisting of paraffins, naphthenes, aromatics and mixtures thereof. 10.The process of claim 1 wherein the diluent is a hydrocarbon in the C8 toC10 range selected from the group consisting of paraffins, naphthenes,aromatics and mixtures thereof.
 11. The process of claim 10 wherein thediluent is a mixture comprising aromatics.
 12. The process of claim 11wherein the diluent is a mixture comprising paraxylene.
 13. The processof claim 1 wherein the diluent is isooctane.
 14. The process of claim 1wherein the first desorbent is a hydrocarbon stream comprisinghydrocarbons in the C5 to C8 range.
 15. The process of claim 14 whereinthe desorbent is a mixture of isooctane and either n-pentane orn-hexane.
 16. The process of claim 1 wherein the second desorbent is ahydrocarbon stream comprising hydrocarbons in the C5 to C8 range. 17.The process of claim 16 wherein the second desorbent is a mixture ofn-hexene and n-hexane.
 18. The process of claim 1 wherein the operatingconditions of the first separation column and second separation columninclude temperatures between 35° C. and 300° C., and pressures between100 kPa and 500 kPa.
 19. The process of claim 1 wherein the operatingconditions of the third separation column and fourth separation columninclude temperatures between 35° C. and 300° C., and pressures between10 kPa and 100 kPa.